net: fix crash in DialIP

Reported by jan.newmarch.

Fixes #1130.

R=r
CC=golang-dev
https://golang.org/cl/2212043

src/pkg/net/iprawsock.go

@@ -288,7 +288,7 @@ func readProtocols() {
 func netProtoSplit(netProto string) (net string, proto int, err os.Error) {
 	onceReadProtocols.Do(readProtocols)
 	i := last(netProto, ':')
-	if i+1 >= len(netProto) { // no colon
+	if i < 0 { // no colon
 		return "", 0, os.ErrorString("no IP protocol specified")
 	}
 	net = netProto[0:i]